By using Jade's language filters directly against the "include" keyword you can write your markdown in a completely separate file and bring it right in for rendering in your templates.
This is cool for a couple of reasons:
You might also want to include external HTML files, for times when you have existing code you want to pull in, and this is competley doable also.
When including HTML files you don't even need to use a language filter. Just use the "include" keyword followed by your file's location and name and you're good to go.